Recycling Carbon Dioxide
April 2014

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Explorer's Biocatalysis Technology Map gives examples of the use of enzymes in carbon capture in a bid to reduce greenhouse-gas emissions. This article describes the progression toward microbial-CO2 fixation that can convert CO2 into valuable products. These valuable products make commercialization of microbial-CO2-fixation systems more appealing to businesses than carbon capture. However, commercial success may take time, and microbial-CO2-fixation systems will have to compete with other CO2-fixation technologies, such as artificial leaves.

Carbon fixation is the conversion of inorganic carbon dioxide (CO2) to organic materials. The main carbon-fixation process in nature is photosynthesis. Most plants perform photosynthesis in order to create energy to grow and live. Photosynthesizing organisms fix approximately 385 billion tons of CO2 annually. However, as a result of the burning of fossil fuels, atmospheric levels of CO2 are the highest they have been since records began. In May 2013, the Mauna Loa Observatory, Hawaii, recorded atmospheric CO2 concentrations of more than 400 parts per million for the first time. This ominous milestone was a significant increase from the 315 parts per million that the observatory measured when it first started recording atmospheric CO2 levels in 1958. Deforestation has exacerbated the situation by removing plants that help in the natural process of carbon fixation. Many scientists predict that further increases in the levels of CO2 will result in significant effects on ocean acidification and the climate. One of many ideas for reducing the increase in atmospheric CO2 is to re-create the carbon-fixation process artificially. The process has the potential not only to reduce atmospheric CO2 but also to create low-carbon alternative fuels and other by-products of commercial interest.

Research

Microorganisms such as microalgae, cyanobacteria, and a number of autotrophic bacteria could provide more efficient ways of CO2 fixation than plants can provide, because these microorganisms grow very quickly. Although promising ideas include the use of such microorganisms to rejuvenate soils with organic carbon substances, the real drive behind optimizing microbial CO2 fixation will come if the process can produce products with commercial value.

Scientists have investigated a number of bacteria that may find use in CO2 fixation and also produce useful substances. One such bacterium is Ralstonia eutropha—a hydrogen-oxidizing bacterium that is present in soils. The bacterium is very adaptable; it has the ability to produce energy by heterotrophic or autotrophic means and can respire aerobically or anaerobically. It is also able to produce polyhydroxyalkanoates (PHAs) when in the presence of an excess sugar substrate. But CO2 may represent a cheaper substrate for manufacturing PHAs using this bacterium. In the presence of CO2, H2, and O2, R. eutropha can produce biomass that contains 80% PHA. PHAs are biodegradable plastics that may replace fossil-fuel-derived polymers if manufacturing costs can fall. Using more efficient PHA-producing bacteria is one way of lowering manufacturing costs.

Scientists can vary the material properties of PHAs by integrating different monomers into the structure, potentially making more desirable products. A team of researchers—based at a number of institutions in Korea, including Myongji University and the Korea Advanced Institute of Science and Technology—are developing methods for incorporating such monomers into PHAs by manipulating the metabolic pathways that R. eutropha uses to create PHAs. The scientists genetically modified the bacteria, causing the bacteria to produce enzymes that accepted a different feedstock in order to create PHA containing 2-hydroxyacid monomers such as lactate. The scientists' accomplishment demonstrates that metabolic engineering of microorganisms will help to improve and adapt these microorganisms to produce desirable materials efficiently. Such developments will increase the likelihood of commercialization of CO2-fixing technology.

Microbial-CO2 fixation also enables the production of biofuels. Researchers at the Indian Council of Scientific and Industrial Research have investigated methods of optimizing CO2 fixation to improve the efficiency of microalgae in producing lipids for biofuels. The team of scientists concentrated on microalgae in the Chlorella genus, which multiply rapidly through highly efficient photosynthesis. In order to maximize the microalgae's availability to and uptake of CO2, the scientists examined the effect of hydroxide ions in the fixation of dissolved CO2 in a medium of Chlorella bacteria. They found that they achieved the highest rate of CO2 fixation with 0.01 mole of sodium hydroxide (or caustic soda). The scientists scaled up the process in a photobioreactor and produced a biomass containing 16.3% lipids, which are the basis for biodiesel. Such research is important for the development of cost-effective and efficient processes for CO2 fixation. As this type of research begins to increase the understanding and efficiency of CO2 fixation, the process will start to realize its potential and stride toward commercialization.

Commercialization

LanzaTech is one company pushing for the commercial reality of CO2 fixation. Currently, LanzaTech uses gas-fermentation technology to turn waste gases into useful substances. For the most part, the company uses microorganisms in a bioreactor to convert waste carbon monoxide from industrial processes into low-carbon fuel and chemicals. At the end of 2012, LanzaTech announced that it would be working with Petronas—Malaysia's national oil company—to evolve LanzaTech's technology to convert waste CO2 to produce acetic acid using the Clostridium ljundahlii bacterium. Acetic acid is an organic compound that finds use in a variety of applications, including in the production of plastics and polymers. According to Grand View Research, the market size for acetic acid in 2013 was 10.5 million tons, and the research company expects the market to grow in value to $12.2 billion by 2020. To create acetic acid, LanzaTech will use hydrogen as an energy source. Although hydrogen is expensive to make, some industries—such as the coal industry—generate excess hydrogen that could find use in the process.

LanzaTech has made some progress with its microbial-CO2-conversion process. In August 2013, the company announced a partnership with the Centre for Advanced Bio-Energy—a center run jointly by the Indian Oil Corporation and India's Department of Biotechnology—to convert CO2 into low-carbon fuels for use in "drop-in" fuels, which are compatible with conventional fuels. The two organizations will use an acetates-to-lipids pathway to convert CO2 to low-carbon fuel. The press release announcing the partnership states, "LanzaTech has developed gas fermentation technology that can directly convert waste CO2 gases into acetates. The Centre for Advanced Bio-Energy is working to increase the production yield of lipids (oils) by 'feeding' acetates to microalgae. The resulting oils can then be refined using a range of existing processing technologies." The partnership demonstrates that LanzaTech feels confident enough in the progression of its CO2-fixing technology to begin exploring its expansion into biofuels production. However, a full-scale system will likely need many years to take shape.

Other companies are moving quickly ahead into large-scale production using CO2-fixing microorganisms. BASF and Corbion-Purac officially formed Succinity GmbH in August 2013 with the aim of producing biobased succinic acid. The food-and-drink industry use succinic acid as an acidity regulator, and polyester manufacturers use it as a precursor. The UK National Non-Food Crops Centre estimates that the annual global production of succinic acid is between 30 000 and 50 000 tons, with a market price of between $6000 and $9000 per ton. Succinity is using Basfia succiniciproducens to fix CO2 and, in March 2014, reported its first commercial quantities of succinic acid. By reaching these quantities in a short time, Succinity is demonstrating the successful commercialization of the technology. However, the speed of progression of CO2-fixing technologies will depend greatly on the microorganism in use and the market potential of the final product.

Concluding Perspectives

The research and development of CO2-fixation processes and products shows encouraging signs of progress, and commercially viable, large-scale systems are starting to become a reality for some products. However, some of the microbial CO2-fixation systems need optimization in areas including bioreactor design. Crucially, a greater understanding of the pathways in microbial-CO2 fixation is necessary to make systems commercially viable and competitive. Developing this fundamental knowledge will enable scientists to fine-tune particular aspects of the process, perhaps using genetic engineering to succeed. Nevertheless, microbial-CO2 fixation will likely receive support from governments and industry alike, because the process can help to decrease carbon footprints, the reduction of which is legally necessary according to the Kyoto Protocol to the United Nations Framework Convention on Climate Change. Companies interested in biofuels and specialty chemicals should watch the development of this technology carefully. The combination of high-value products, use of waste industrial gases, and benefits to the environment makes the technology very appealing.

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